The present invention relates to a regulator, and more particularly, to a Pulse Width Modulation (PWM) controller for a buck and boost converter.
A regulator is used in a power supply circuit to obtain a desired output voltage from an input voltage. Examples of regulators include a step-up, or boost converter, a step-down, or buck converter, and a buck and boost converter. A boost converter generates output voltage that is higher than the input voltage. A buck converter generates output voltage that is lower than the input voltage. A buck and boost converter implements the functions of both a buck converter and a boost converter.
One of the important characteristics of a regulator is the linearity of the output voltage in order to obtain a stable voltage. Normally, the regulator includes an error amplifier, a pulse width modulation (PWM) controller, and an output transistor. The error amplifier monitors the output voltage of the regulator and generates an error voltage, which is in accordance with a difference between the output voltage and a reference voltage. The PWM controller generates a pulse having a variable duty ratio, which is in accordance with the error voltage. The pulse drives the output transistor and adjusts the level of the output voltage. That is, the value of the output voltage is dependent on the duty ratio of the pulse, which activates and deactivates the output transistor.
FIG. 1 shows an example of a conventional buck converter 10. The buck converter 10 includes an error amplifier 12, a PWM controller 14, and an output transistor 16. The output transistor 16 is formed by an NMOS transistor. The error amplifier 12 generates an error voltage VERR, which is in accordance with the difference between a feedback value VFB of the output voltage VOUT and a reference voltage VEREF. The PWM controller 14 includes a current source 18, a saw wave generator 20, and a comparator 22. The current source 18 generates current I1, which is in accordance with a reference voltage VPREF. The saw wave generator 20 receives a clock CL1, deactivates a transistor 24 to charge a capacitor 26 with the current I1, and activates the transistor 24 to discharge the capacitor 26. As shown in FIG. 2, the saw wave generator 20 repeats the charging and discharging of the capacitor 26 in predetermined cycles and generates a saw wave VS1, which has a constant voltage amplitude VP1, based on the current I1. The comparator 22 compares the saw wave VS1 with the error voltage VERR to generate a pulse P1, which drives the output transistor 16. When the error voltage VERR is greater than the saw wave VS1, the pulse P1 has an H logic level (or active high), which activates the output transistor 16.
In the buck converter 10 of FIG. 1, the duty ratio D1 of the pulse P1 may be expressed by the equation shown below.D1=VERR/VP1  Equation 1Accordingly, the transfer function of the buck converter 10 may be expressed by equation 2 when using equation 1.VOUT=VIN×D1=VIN×VERR/VP1  Equation 2
In equation 2, VIN/VP1 may be regarded as a constant. Accordingly, the output voltage VOUT of the buck converter 10 varies linearly in accordance with the error voltage VERR.
FIG. 3 shows an example of a conventional boost converter 30. The boost converter 30 includes an error amplifier 32, a PWM controller 34, and an output transistor 36. The output transistor 36 is formed by an NMOS transistor. In the boost converter 30, a current source 38 of the PWM controller 34 generates a current I2 in accordance with an error voltage VERR generated by the error amplifier 32. A saw wave generator 40 controls a transistor 44 with a clock CL2 and to repeatedly charge and discharge a capacitor 46 in predetermined cycles.
Referring to FIG. 4, a saw wave VS2 is generated by the saw wave generator 40. The saw wave VS2 has a variable voltage amplitude VP2, based on the current I2, which varies in accordance with the error voltage VERR. A comparator 42 compares the saw wave VS2 with a reference voltage VPREF to generate a pulse P2, which drives the output transistor 36. When the voltage of the saw wave VS2 is higher than the reference voltage VPREF, the pulse P2 has an H logic level (or active high), which activates the output transistor 36.
In the boost converter 30 of FIG. 3, the duty ratio D2 of the pulse P2 may be expressed by equation 3 shown below.D2=1−VPREF/VP2  Equation 3Accordingly, the transfer function of the boost converter 30 may be expressed with equation 4 (below) when using equation 3.VOUT=VIN/(1−D2)=VIN×VP2/VPREF  Equation 4In equation 4, VIN/VPREF may be regarded as a constant. Further, VP2 is a value proportional to the error voltage VERR. Thus, the output voltage VOUT of the boost converter 30 varies linearly in accordance with the error voltage VERR.
Accordingly, the output voltage VOUT of the buck converter 10 shown in FIG. 1 and the output voltage VOUT of the boost converter 30 shown in FIG. 3 are both linearly controlled. However, the linear control differs between the PWM controller 14 of the buck converter 10 and the PWM controller 34 of the boost converter 30. Thus, when applying the PWM controller 34 (equation 3) to the buck converter 10 (equation 2), the output voltage VOUT cannot be linearly controlled. Likewise, when applying the PWM controller 14 (equation 1) to the boost converter 30 (equation 4), the output voltage VOUT cannot be linearly controlled. Accordingly, the PWM controller 34 is not suitable for use in a buck converter, and the PWM controller 14 is not suitable for use in a boost converter. That is, the conventional PWM controllers 14 and 34, which perform linear control differently, are not suitable for use in a combined buck and boost converter.
It would be advantageous to have a PWM controller suitable for linear output control in a buck and boost converter.